1. Field of the Invention
This invention relates to a silver catalyst to be used in the production of ethylene oxide by catalytic vapor-phase oxidation of ethylene with molecular oxygen and to a method for the production of the catalyst.
2. Description of the Prior Art
The catalyst used in the production on a commercial scale of ethylene oxide by catalytic vapor-phase oxidation of ethylene with molecular oxygen is required to possess high selectivity, high activity, and a long service life as essential qualities thereof.
Various studies have been made to date on the preparation of the catalyst with a view to improving the qualities thereof as required. Enormous efforts have been made in the improvement of the carrier, the reaction promotor, and the silver compound. A number of reports which deal with carriers have been published. These include the published specifications of U.S. Pat. No. 3,207,700; Japanese Patent Publication Nos. 43-13,137(1968), 45-21,373(1970), 45-22,419(1970), and 45-11,217(1970); Japanese Pat. Laid-Open No. 57-171,435; U.S. Pat. Nos. 2,766,261, 3,172,893, and 3,664,970. Most of the inventions disclosed therein pertain to micropore distribution and specific surface area of the carrier.
U.S. Pat. No. 2,125,333 discloses an alkali metal salt containing sodium or potassium and a metal salt thereof used as an additive to the silver catalyst for use in the production of ethylene oxide.
U.S. Pat. No. 2,238,474 discloses that sodium hydroxide improves the activity of a catalyst for the production of ethylene, whereas potassium hydroxide exerts an adverse effect on the catalytic action.
U.S. Pat. No. 2,765,283 discloses that a silver catalyst is improved by the addition of an inorganic chloride such as sodium chloride in an amount in the range of from 1 to 2,000 ppm to a catalyst carrier before silver is deposited on the carrier.
U.S. Pat. No. 2,799,687 discloses that a halide such as sodium chloride or potassium chloride present in an amount in the range of from 20 to 16,000 ppm acts as an inhibitor and causes a degradation in the activity of a catalyst.
U.S. Pat. No. 4,007,135 discloses a catalyst for the production of alkylene oxide which contains in the carrier thereof copper, gold, zinc, cadmium, mercury, niobium, tantalum, molybdenum, tungsten, and vanadium, desirably chromium, calcium, magnesium, strontium and/or more preferably vanadium, and equally preferably an alkali metal in an amount exceeding the amount in which they exist naturally in the carrier in the form of impurities or cement and which possess the ability to promote catalysis.
U.S. Pat. No. 4,168,247 discloses a catalyst for the production of alkylene oxide which has silver contained in a porous refractory carrier with a specific surface area in the range of from 0.05 to 10 m.sup.2 /g and which further contains in the carrier at least one alkali metal selected from the group consisting of sodium, potassium, rubidium, and cesium in an amount exceeding the amount in which they exist naturally in the carrier in the form of impurities or a binding agent and which promotes catalysis.
U.S. Pat. No. 4,278,562 discloses a catalyst for the production of alkylene oxide produced by first applying silver and optionally sodium or lithium in the form of corresponding salts to a carrier, heating the carrier and deposited salt in the manner generally practised, and thereafter applying thereto alkali metals such as potassium, rubidium, and cerium in the form of salts thereto in combination with an amine and/or ammonia.
Japanese Pat. Laid-Open No. 55-145,677(1980) discloses a silver catalyst for the reaction of oxidation which contains alumina, silica, and titania in a combined amount of not less than 99% by weight and metals of Groups Va, VIa, VIIa, VIII, Ib, and IIb in the Periodic Table of the Elements in the form of metal oxides in a total amount of less than 0.1% by weight and which has silver and optionally an alkali metal component or alkaline earth metal component deposited on a non-oxidizing carrier incapable of assuming an oxidizing color on exposure to methyl red of pKa of +4.8.
U.S. Pat. No. 4,368,144 discloses a silver catalyst for the production of ethylene oxide which comprises an alpha-alumina carrier having a sodium content of not more than 0.07% by weight and a specific surface area in the range of from 0.5 to 5 m.sup.2 /g, 5 to 25% by weight based on perfected catalyst of metallic silver particles deposited on the carrier, and 0.001 to 0.05 gram equivalent per kg of perfected catalyst of at least one alkali metal or alkali metal compound contained in the carrier in addition to the amount in which the alkali metal or alkali metal compound naturally exists in the carrier.
Japanese Pat. Laid-Open No. 56-105,750(1981) discloses a silver catalyst for the production of ethylene oxide which is obtained by preparing a carrier consisting mainly of alpha-alumina, having a sodium content of not more than 0.07% by weight, and possessing a specific surface area in the range of from 1 to 5 m.sup.2 /g, subjecting the carrier to an immersing treatment in an immersing liquid having 0.001 to 0.05 gram equivalent per kg of perfected catalyst of a complex of an alkali metal with boron, a complex of an alkali metal with molybdenum, and/or a complex of an alkali metal with tungsten incorporated in a decomposing silver solution such that the ratio of deposition of silver falls in the range of from 5 to 25% by weight based on the amount of the perfected catalyst, and subjecting the impregnated carrier to thermal reduction or thermal decomposition.
Japanese Pat. Laid-Open No. 57-107,241(1982) discloses a silver catalyst for the production of ethylene oxide which contains at least sodium (Na) as a cationic component and chlorine (Cl) as an anionic component besides silver in amounts such that the atomic ratio of Cl/Na is less than 1.
U.S. Pat. No. 4,415,476 discloses a silver catalyst for the production of ethylene oxide which contains at least sodium and cesium as cationic components and chlorine as chloride as an anionic component in addition to the silver.
E.P. 247414 discloses a silver catalyst for the production of ethylene oxide which contains at least sodium, potassium, rubidium, and/or cesium as cationic components in addition to silver as a catalytic component and uses a carrier consisting mainly of alpha-alumina and having a surface area in the range of from 0.6 to 2 m.sup.2 /g, an absorption ratio in the range of from 20 to 50%, a silica content in the range of from 0.5 to 12% by weight, a silica content in the range of from 0.5 to 12, preferably from 1 to 8, (weight %/m.sup.2 /g) per surface area, and a sodium content in the range of from 0.08 to 2% by weight.
Numerous pertinent reports have been published as demonstrated above and most of them are directed to enhancing the catalytic qualities of a silver catalyst by the addition of alkali metals falling in a limited range to the silver catalyst. Though the catalysts disclosed therein exhibit ideal initial catalytic properties, they are deficient in terms of service life.
The carriers used in the silver catalysts for the production of ethylene oxide have many problems not yet elucidated and required to be further improved. For example, the choice of composition of the components for the carrier, the physical properties of carrier such as specific surface area, pore diameter, pore size distribution, pore volume, porosity, particle diameter, and particle shape, and the chemical properties of such carrier materials as alpha-alumina, silicon carbide, silica, and zirconia remain yet to be improved for optimization.
An object of this invention, therefore, is to provide a novel silver catalyst for the production of ethylene oxide which combines high selectivity, high activity, and a lasting service life.
Another object of this invention is to provide a novel silver catalyst for the production of ethylene oxide which enjoys an enhanced service life owing to the use of an alpha-alumina carrier having a specific surface area in the range of from 0.75 to 5 m.sup.2 /g and an apparent porosity in the range of from 45 to 70%.
The present inventors have been pursuing a study concerning the composition of components and the specific surface area of an ideal carrier to be used in a silver catalyst for the production-of ethylene oxide. As disclosed in U.S. Pat. No. 5,077,256, they have already developed a silver catalyst which is able to exhibit high selectivity and maintain this high selectivity for a long time by using an alpha-alumina carrier having the outer surface thereof and the surface of the pores therein coated with amorphous silica. After further continuing the study, they have found that a silver catalyst using an alpha-alumina carrier having the outer surface of carrier and the surface of pores therein coated with an amorphous silica-alumina mixture exhibits higher selectivity and maintains this higher selectivity for a longer time than the silver catalyst using the amorphous silica coating on the outer surface of carrier and the surface of pores therein. This invention has been perfected as a result.